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h4tt3n

I've made pure iron that doesn't rust, even in salt water (photos)

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Hello,

 

I would like to show you this interesting phenomenton I came across by pure chance. The attached photos show a slab of pure iron that I have made from bog iron ore in a clay furnace, exactly like iron was made several thousand years ago in the european iron age. The slab is approx. 21 cm (8.3 in) wide and 6 cm (2.4 in) high and has been cut out of a larger bloom weighing about 7.5 kg (16.5 lbs) with an angle grinder.

 

I have had it lying outside the entire summer in an iron age heritage center as part of a display on prehistoric ironsmelting. Now, after some time I noticed that the iron slab did not rust, even though we've had quite a bit of rain. Parts of the slab remained fresh and shiny as the day I cut it. Out of curiosity I threw it in a wooden tub of water, where it stayed for several days, still with no sign of rust. Finally I threw it in salt water roughly as salty as sea water to force start rust formation.

 

As you can see in the photos, it didn't work. Just prior to capturing these photos I've taken the slab out of the salt water and gently brushed it over with a soft cloth, nothing more. Parts of the iron slab still shines with a silvery shimmer, completely unaffected by the corrosive environment. The rest is covered with a thin, dark brown to black film that can easily be rubbed away. The reddish brown sections is iron silicate slag that got stuck inside the bloom during its formation in the furnace.

 

Could someone here please explain how come this particular block of iron does not rust?

 

Mind you, as opposed to stainless steel this is very pure iron without any alloy elements, except perhaps for a little bit of phosphorus and/or carbon.

 

Also, doesn't this have some interesting application possibilities in the field of engineering?

 

Thanks in advance,

 

Mike

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Edited by h4tt3n

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There's no way that iron smelted " from bog iron ore in a clay furnace, exactly like iron was made several thousand years ago" is anything like pure iron.

Different concentrations of impurities in different parts of the metal could mean that some parts are protected from rusting in much the same way that anodic protection or galvanising work.

 

It's also possible that chromium adventitiously present is reducing the tendency to rust.

it's interesting though.

Do you know anyone who could get it analysed for you?

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Iron made from bog ore will often contain residual silicates, which can form a glassy coating that grants some resistance to rusting.

 

http://en.wikipedia.org/wiki/Bog_iron

 

I was honestly thinking it was something acting as a sacrificial anode. Might be some combination of both but sounds like silicates are the most probable cause.

Edited by Endy0816

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The silicates might have been plausible, right up until he cut through them with an angle grinder.

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Could it contain a significant amount of phosphate?

 

http://en.wikipedia.org/wiki/Stainless_steel

 

A few corrosion-resistant iron artifacts survive from antiquity. A famous example is the Iron Pillar of Delhi, erected by order of Kumara Gupta I around AD 400. Unlike stainless steel, however, these artifacts owe their durability not to chromium but to their high phosphoruscontent, which, together with favorable local weather conditions, promotes the formation of a solid protective passivation layer of iron oxides and phosphates, rather than the non-protective cracked rust layer that develops on most ironwork.

 

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Hello John,

 

Well okay, considering that my "laboratory" consists of a grass field with clay furnaces, and ill defined bog ore - and no measuring equipment whatsoever - my understanding of the term "pure" may differ a bit from that of a scientist, I'll grant you as much. Having said that, I think you might be surprised how pure the iron made in this process actually is according to analyses. As a blacksmith I have come to prefer the home made iron over at least some of the cheaper irons available on the market. It is more ductile, easier to forge weld, and can withstand beeing worked at both lower and higher temperatures than off-the-shelf bar iron from the local hardware store. Of course, none of this matters if you are just going to electro-weld it into a grid and cast it into a concrete element, but it matters if you intend to really work with the material.

 

Among fellow blacksmiths it is a common saying about iron and steel that the older it is, the better the quality. As far as I know, this is mainly due to the ever-increasing amount of poorly sorted scrap beeing meltet into virgin iron before shipping. That's good for the environment, but bad for steel quality. It cracks and breaks easily upon forging, and melts or burns easily upon heating. I have often seen - and heard from other blacksmiths - how chunks of un-molten high-alloy steel appears inside raw steel bars, ruining both the tools used and the project at hand. So again, which material is the purer one?

 

As for the lack of rust, I think your theory about anodic protection might at least in part explain what is going on. At least when it comes to the salt water experiment because of the presence of a good electrolyte. Still, does this explain how it didn't rust while laying outside for six months directly exposed to the weather? Rain water is a poor conductor of electricity, and thus a poor electrolyte. Also, the iron slab was often moistened by rain, but never submerged (we get a lot of rain, but not that much). I know for a fact that this metal does not contain even the slightest trace of chromium, so it has to be something else.

 

I'm meeting with some colleagues this weekend, mostly smiths and archaeologers, to do some more experiments. I'll see if I can get one of them to do an analysis of the metal, aiming specifically for an explanation why it resists corrosion.

 

Cheers,

Mike

Edited by h4tt3n

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The silicates might have been plausible, right up until he cut through them with an angle grinder.

 

Would they be only on the surface or distributed throughout?

 

Might be an idea to test conductivity depending on what resources you have available.

 

 

I do like the idea of sticking a glaze on metals though. Biological iron production is a decent thought too.

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Hi h4tt3n, nice to see you again!

 

I know no explanation for this astounding observation...

 

Ultra-pure iron resists oxidation under some conditions. Unattainable by the described methods.

Silicates and phosphates maybe, but I suppose in big proportion only. 1.8% Si still permits to rust but prevents to forge.

Quite pure iron-nickel and iron-cobalt rust very little, like Permalloy and similar ones, despite containing no Cr.

 

Is your iron ferromagnetic?

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Hello everyone,

 

Will try to answer some of the questions that have come up...

 

@Moontanman

 

Yes, it is quite probable that this iron contains phosporus. There is often quite a lot of Phosphates in the ore, 1-10% is not uncommon. So, the anti-rust mechanism we see here may be the same as in the Delhi Pillar. What makes this interesting is that I know how to produce the material.

 

@Endy0816

 

Raw iron bloom contains some silicates that come from the ore, but I don't think they play a large role here. The slag is very brittle and porose, and it doesn't cover the iron particles. If they did, I wouldn't be able to forge it. Been there, done that - when the metal is hot, it's like trying to forge porridge.

 

@Enthalpy

 

Hi there, and thanks! I know for sure that this iron contains absolutely no silica, nickel, cobalt, or chromium - not even the slightest trace. Also no manganese, although this metal is abundantly present in the local ore. The smelting process is happening at such low temperatures, 800-1300 degrees celsius, that other metals cannot be smelted.

 

And yes, it is very ferromagnetic - it's almost pure ferrite.

 

The explanation of the anti-rust mechanism must be somewhere else.

 

Cheers,

Mike

Edited by h4tt3n

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Hello again,

 

Now it's been more than an month since I started this post. In the mean time the iron slab has been lying in a bucket of salt water in the garden, fully exposed to wind and weather. As you can see in the images, there is still no sign of rust. Most of the surface is covered in a thin, black film that can be rubbed off with a cloth. As before, a considerable part of the iron is still shining like fresh polished silver, with no sign of rust, oxidation or corrosion of any kind.

 

I still haven't found any satisfying explanation to the phenomenon.

 

Cheers & merry christmas,

Mike

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Edited by h4tt3n

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Amazing!

 

Mild steel containing very little carbon is known to resist corrosion better (but a month in brine?) - though, could your production process make low-carbon steel?

 

One other direction would be an alloying element that precipitates carbon, but I've read it for small amounts of carbon.

 

Silicon would have better reasons to be in your steel and is known to help against corrosion. Easy to test: from 1.8%Si up, the steel is impossible to forge.

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Hi Enthalpy,

 

With the iron-age process it is indeed possible to produce very low-carbon iron - almost pure ferrite. So that's a possibility, yes.

 

I don't know about precipitation of carbon in iron - will have to investigate on that one.

 

I know of the properties of steel with silicon, but generally these alloys aren't forged for the very reason you mention. I have seen several analyses of both pre-historic iron and modern iron made with prehistoric methods, and I've never seen any trace of silica in them. The metal often contains silicates in the form of slag, but that's an entirely different story. There simply isn't heat enough in the smelter (800-1300 degrees celsius) to reduce silicates into free metallic silica.

 

In the museum where I work as a blacksmith there's an iron age knife on display with the exact same smooth black surface as the iron slab in the photos. No thrace of rust or corrosion, nothing's missing, it's just shiny black. It's from the first centuries AD, making it at least 1800 years old. When I first saw it I thought it was bronze, which in the right conditions can be perfectly preserved. When I talked to the archaeologer supervising the dig, he told me that it was indeed iron, and that he was as baffled about it's extremely fine condition as I was.

 

Things like these just pokes my curiosity!

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Hello, h4tt3n, a most interesting post.

 

Your smelting process sounds similar to that employed in the Blackdown hills in Somerset from the (pre Roman) iron age to medieval times.

 

I'm not sure how thick you ingot was but it seems a bit thicker than they could manage.

 

You say this iron is soft and ductile,

 

Lump mass of cast iron is relative insensitive to rust and survives for centuries as bollards etc on sea walls.

 

But cast iron is brittle, not ductile.

 

A structural factor that promotes corrosion is lack on homogeneity so perhaps your product is particularly homogenous?

Have this happened to many samples.

 

After the break I will look out our blackdown local experts to compare notes.

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The well-preserved old knife contains certainly much carbon (or at least hardening elements), as pure ferrite wouldn't stay any sharp. So there's an other reason.

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I think it's about time I explain the process in a bit more detail. In the attached pictures you can see the process from raw ore to finished object.

First, the bog iron ore is dug out of the ground. It has a very rusty color, ranging from greyish black over brown to yellowish red. The ore is roasted on a fire to make it porose, and is then crushed to a fine powder. The crushed ore is put into a smelter with charcoal, usually in a 1 / 1 ratio. Air is blown into the bottom of the smelter to increase temperature. Usually, the smelter is about 700-800 degrees celsius in the top (glowing a dull red) and 1200 - 1300 degrees in the bottom (glowing an incandescent yellowish white). Iron oxide is reduced to metallic iron by carbon monoxide, and the remains of the ore is molten into an iron-rich slag, which protects the iron from re-oxidation and taking up too much carbon. When the slag has filled up the bottom of the smelter, it is tapped from the furnace so it won't clog up the air inlet. Please note that the iron does not melt. At no point in the process from ore to tool is the iron liquid. Once the so-called bloom has grown big enough, the process is stopped and the iron extracted from the smelter. The bloom is then usually forged on a large rock by 2-3 people with sledge hammers. This consolidates the iron, removes excess slag, and shapes the iron so it's easier to work with. Finally, the bloom is cut into two or three "klimps" each weighing 1-2 kilograms. Finally, the klimps are heated in the forge and worked into bars, tools, or weapons.

 

Cheers,

Mike

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Hi Studiot,

 

Indeed this method is very similar to the (many different) ones used in Britain and northern Europe throughout the iron age.

 

Formally, the iron lump made in this process is called a bloom, whereas ingot refers to something than has been molten.

Liquid cast iron or pig iron is a result of the "modern" direct iron production method and was unknown in European prehistoric times.

 

Bloomery iron has not been liquid at any time and is very, very different from cast iron.

 

Bloomery iron is usually very heterogene, consisting of ferrite crystals of very different size with quite a lot of slag inclusions in it.

 

I'll look into this Blackdown place, sounds interesting. Maybe we could do a smelt together someday :)

 

Cheers,

Mike


Hi Enthalpy,

 

Carbon content is a difficult one. We know that quenched, tempered steel was used in northern Europe deliberately sometime in the middle iron age onwards, but the process wasn't common everywhere before early middle ages. Unfortunately, the preservation process used on iron artefacts include heating the object to a glowing red, thereby forever erasing any detail about how the metal was treated. Also measuring the carbon content of an artefact is a destructive process, since you need to cut out a piece, grind it and do microscopy on it. So it's not done as often as one could wish for.

 

In the iron age there were other ways to make a knife hard than by using steel. One is by using phosphoric iron, which gets very hard and brittle, similar to carbon steel, but harder to work with. Phosporus is very common in bog iron ore, and all the iron I have ever made contains phosporus. Another way is to cold-hammer the edge of a soft iron knife. This deforms the structure of the iron and makes it harder and more brittle. This is used both for thinning and sharpening, and was commonly used on schythes up until about 60 years ago.

 

So, long story short, I simply don't know the carbon content of the knife, it was never analysed, but it is very likely that there isn't any carbon in it.

 

Cheers,

Mike

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No carbon could be a help against corrosion.

 

About analysis: some methods make virtually no damage to the artifact. Modern analysis apparatus evaporate a tiny bit of the alloy using a spark that also lets the vapour emit light; the spectrum tells the composition. Works for exposed parts, to be repeated at several places because alloys are inhomogeneous. The apparatus costs a few k€ but users or manufacturers could run theirs on your artifacts.

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Hi Enthalpy,

 

Yes, that is perfectly true. As far as I know, this is in fact one of the method used by archaeometallurgists. The thing is, prehistoric and medieval iron - having never been molten - often contains many layers of different iron alloys. Knives and axes were deliberately forged in soft iron, and then a piece of hard steel or phosphorus iron were welded into the edge. Also, old tools and weapons were recycled again and again, so that f.inst. inside an axe you may find the remains of an older sword or other artefact of importance. The point is, you can't just do one surface analysis and then conclude that the entire artefact is of that particular alloy. You would risk to lose invaluable information about the history of the object and the process of crafting it. Therefore, artefacts are usually cut up, and then both microscopic and evaporative analysis is done on each layer - both metal and slag inclusions - is done seperately. But apart from that, for modern, completely homogenous steel, the method you describe would be perfectly fine :)

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Hello all,

 

Here's an update on the iron slab. These photos were taken one month ago:

 

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And these photos were taken today:

 

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As you can see, there is practically no change. Still no rust. There's also still non-corroded metal on the surface, although the areas with shiny metal seems to shrink very slowly. Salt has been added regularly to keep water salinity high in the container.

 

Cheers,

Mike

Edited by h4tt3n

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The Iron in question might of came from the moon believe it or not, probably blown off the face of the moon during a meteor impact, and landed on earth long ago, it has been noted that both NASA and Russia have found Iron on the moon that does not rust even in salt water that has no extra silica chromium carbon etc etc that we would tend to see provide anti rust properties. Hope this helps in your search.

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It's a big discovery, I think. With such low percentage of phosphorus and carbon , can iron be turned into steel??? Confused...

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Hi - interesting read....

 

I would like to comment on the insanity of smelting iron in bare feet - lol. It is probably fine and I am just ignorant of the proper safety procedures... but I would no way do what that bloke is doing in those pics without boots on.

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If the object is submerged in brine, you maybe shouldn't be expecting to see the typically scaly orange rust.

 

If it's sufficiently anaerobic, the Schikorr reaction may well be in play which may give a degree of passive protection via a magnetite surface layer. The 'thin black film' you mention is also suggestive of this.

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